Drive circuitry for light emitting film displays

ABSTRACT

Driving circuitry for multiplexed light emitting film displays is shown wherein the unselected segment electrodes of a segment matrix and/or the unselected digit electrodes are connected together to prevent establishment of floating complex seriesparallel capacitance networks which could otherwise drive certain of the unselected electrodes to emit unwanted light. Also, the polarity of the voltage applied to inductors forming part of the electrode drive circuits is reversed during each segment drive cycle to eliminate decrease of light intensity of a respectively fired electrode due to energy losses incurred in the inductors and in the display.

United States Patent Nelson Sept. 3, 1974 DRIVE CIRCUITRY FOR LIGHTEMITTING FILM DISPLAYS Alan H. Nelson, 4232 La Concetta Dr., YorbaLinda, Calif. 92686 Filed: Aug. 20, 1973 Appl. No.: 390,097

Inventor:

US. Cl 315/169 TV, 340/324 R, 340/336 Int. Cl. H05b 33/00 Field ofSearch 3l5/l69 TV, 169 R, 284;

340/324 R, 336, I66 EL References Cited UNITED STATES PATENTS Morgan3l5/l69 R I-libi 3l5/l69 R Nishizawa 3l5/l69 TV 3,786,307 l/l974Robinson 3l5/l69 TV Primary ExaminerJohn S. Heyman [57] ABSTRACT Drivingcircuitry for multiplexed light emitting film displays is shown whereinthe unselected segment electrodes of a segment matrix and/or theunselected digit electrodes are connected together to preventestablishment of floating complex series-parallel capacitance networkswhich could otherwise drive certain of the unselected electrodes to emitunwanted light.

Also, the polarity of the voltage applied to inductors forming part ofthe electrode drive circuits is reversed during each segment drive cycleto eliminate decrease of light intensity of a respectively firedelectrode due to energy losses incurred in the inductors and in thedisplay.

16 Claims, 6 Drawing Figures PATENTEB SHEEI 1 OF 3 llalllllll'lllllDRIVE CIRCUITRY FOR LIGHT EMITTING FILM DISPLAYS BACKGROUND OF THEINVENTION This invention relates to electroluminescent displays and hasparticular reference to drive circuitry for light emitting filmdisplays.

Light emitting films are well known in the art for use as data displaysor the like and are disclosed, for exari1 ple, in the U.S. Pat. toGordon Steele et al, US. Pat. No. 3,560,784, issued on Feb. 2, 197 lSuch films comprise basically a polycry-stalline phosphor film or thelike having electrode layers on opposite sides thereof, at least one ofwhich is transparent. The assembly forms a capacitor and when a voltageof sufficient potential is applied across the electrode layers, thephosphor film will glow in the area between the opposite energizedelectrodes and will be visible through the transparent electrode.

Light emitting films of the above type, when used for data displays,have certain characteristics which require special driving circuits forenergizing the same. For example, it has been found that application ofan appreciable DC voltage potential across the electrode layers tends todegrade the life of the film. Also, in digital displays of either thesegment matrix or the dot matrix type, a relatively large number ofsegment electrodes (in the case of a segment matrix) or a large numberof X-Y electrodes (in the case of a dot matrix) are required,particularly for a multi-digit display. Also, a large number ofindependent drive circuits are required to drive such electrodes. Thisnumber is reduced to some extent by driving the array of electrodes at ahigh rate in a multiplexed fashion wherein only one digit is formed at atime, with the persistence characteristic of the film and thepersistence of vision of the eye eliminating flickering.

In prior art driving circuits of the above type, the various unselectedelectrodes are found to form a number of series and parallel capacitorcombinations in circuit with a selected electrode pair and thus when theselected pair is driven to the proper voltage necessary to cause thesame to emit a required amount of light, relatively high voltages arealso supplied to the unselected pairs, causing unwanted light emissionby at least some of those unselected pairs. Thus, in order to prevent avoltage build-up in unselected electrode pairs sufficient to causeunwanted light emission, it has been necessary heretofore to limit thenumber of electrodes which may be connected in parallel.

Also, because of the large number of drive circuits required (even in amultiplex system) for driving the different electrodes of a data displayunit, economy of circuitry is of paramount importance. Heretofore,although relatively simple drive circuits have been proposed, those ofwhich applicant is aware result in a considerable amount of DC voltagebeing applied to the electrodes which tends to damage the film and/orelectrodes and thus reduce its effective life. Efforts to eliminate thistendency have resulted heretofore in complex and expensive circuitry.

SUMMARY OF THE INVENTION It therefore becomes a principal object of thepresent invention to provide a drive circuitry for a multi-digit lightemitting film display in which capacitive networks tending to driveunselected electrodes to illumination are essentially eliminated.

Another object is to reduce the complication, and consequently the cost,of drive circuitry for light emitting film displays of the above type.

Another object is to maintain full illumination of different energizedelectrodes of a light emitting film display of the above type regardlessof the amount of repetitive usage of such electrodes.

Another object is to increase the operating life of light emitting filmsof the above type.

According to a preferred embodiment of the present invention, eachelectrode is connected in circuit with a respective inductor to form aresonant circuit, the inductors and corresponding electrodes beingconnected together through low impedance switching means so as tomaintain all unselected electrodes at a fixed reference voltage at alltimes.

Also, preferably, a switching device alternately connects the inductorsof the segment electrodes to voltages of different potentials so thatenergy stored in the inductors of both selected and unselected segmentsis always decaying at the same rate during the time interval when asegment electrode may be energized.

Other individual switching devices for both the segment electrodes andthe digit electrodes are provided and are normally held on (conducting)so that the voltage applied across the inductors is such that the totalenergy stored in such inductors is equal to the energy required to driveeach electrode pair to their required peak voltage necessary to causefull light emission. Now, if the two such individual switching devicesin circuit with a selected electrode pair and their respective inductorsare turned off simultaneously, the magnetic fields of such respectiveinductors collapse, causing the resonant circuits comprising theinductance and capacitance formed by theelectrode pair to resonate orring at a frequency determined by the inductance L of the inductors andthe capacitance C of the electrode pair, i.e., f =1 /21r V LC.

Since each of the unselected electrodes is connected to a commonreference voltage and only one digit array is selected at a time, thecapacitances thereof cannot be combined in different complexseries-parallel paths which could otherwise cause unwanted lightemission of such unselected pairs. That is, the voltage applied to theunselected pairs will remain constant and well below that required toluminesce. Furthermore, only a negligible DC component will be appliedacross the electrode pairs.

Also, since relatively different voltage potentials are alternatelyapplied to the inductors of the segment electrodes, the energy stored insuch inductors is always decaying during the time interval when asegment electrode may be energized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates diagramatically amultiplexing arrangement of data display light emitting films embodyinga preferred form of the present invention.

FIG. 2 illustrates a display panel including a typical FIG. 5 is adiagram showing the waveforms at specific points in the circuit of FIG.1.

FIG. 6 is a schematic diagram of a modified form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 2 and 3in particular, a multidigit display panel, generally indicated at 11, isillustrated. Although only three digit arrays 12, 13, and 14 are shown,it should be understood that a greater number of such arrays may beincorporated in the panel, for example, twelve.

The display panel, which per se forms no part of the present invention,may comprise a glass substrate 15 (FIG. 3) to which are bonded a seriesof separated transparent electrodes 16 in a common layer or plane. Theelectrodes 16 are located side-by-side and form the digit electrodes. Atransparent polycrystalline phosphor film 17 is interposed between adielectric film 18 and a second transparent dielectric film 19, thelatter being bonded to the surfaces of the digit electrodes 16. A seriesof separated segment electrodes 20 in a common layer, which may beopaque, are bonded to the dielectric layer 18 and are arranged in groupsof eight, overlaying respective digit electrodes 16. Seven of suchsegment electrodes 20 form the figure 8 as seen in FIG. 2, and theeighth which may or may not be used forms a decimal point 21.

Each segment electrode 20, together with its respective digit electrode16, form what I will term an electrode pair having a capacity C which,when an AC voltage in the neighborhood of 350 volts, peak or 700 voltspeak to peak, is impressed thereacross causes the interposed portion ofphosphor film 17 to emit visible light. Where digits having a height of0.35 inches are incorporated in the panel 11, the total capacitance persegment is approximately 48 pf. (pico farads) for horizontal segmentsand 60 pf. for vertical segments.

As seen in FIG. 1, an inductor 22 of approximately 50 mh. (millihenries)is connected between each digit electrode 16 and a common line 29. Also,the corresponding segment electrodes 20 of the different digit arrays12, 13, 14, etc., are connected together through respective lines 23 andeach such line is connected through an inductor 24 of approximately 50mh. to a switching device shown generally at 39, capable of alternatelyconnecting all of the inductors 24 to the positive side of a DC powersource 39b of one potential (V,) and to the negative side of a DC powersource 39a and 3912 are connected to the common line 29.

PNP transistors 26, forming digit switching devices, are connectedbetween respective digit electrodes 16 and a third DC power supplysource 27. A diode 28 is connected in series with the collector of eachtransistor 26 for a purpose to be described later. Likewise, an NPNtransistor 30 is connected between each segment line 23 and the commonline 29. A diode 32 is connected in series with the collector of each ofthe transistors 30.

The use of PNP transistors for digit drivers and NPN transistors forsegment drivers is arbitrary. NPN digit drivers and PNP segment driverswould function as well.

Normally, all of the transistors 26 and 30 are biased to on orconducting condition, causing a current flow through all of theinductors 22 and 24, thereby 26 and 30 are momentarily turned off bysimultaneously applying proper biasing signals 138 and 139 (FIG. 5) tothe bases e and e.,, respectively, thereof concurrently with switchingof the device 39 from source 39b to source 39a as seen at 40 (FIG. 5).This will induce a voltage having a waveform 49 (FIG. 5) in the selectedresonant circuits and will drive the selected main digit electrode 16and secondary segment electrodes 20 to light emission. If L l/(21rf)where f is the desired drive frequency and C is the equivalentcapacitance of the selected electrode pair comprising a digit electrode16 and segment electrode 20, the connected resonant circuit will ring atthe desired frequency and amplitude to cause full light emission.

The optimum potentials of the power supplies 27, 39a and 39b, depend onthe resistances of the inductors 22 and 24 and the voltage drop acrossthe transistors 26 and 30 and diodes 28 and 32. In the presentembodiment, the voltage drop across the transistors 22 and 24 plusdiodes 28 and 32 is approximately 0.9 volts and the voltage drop acrosseach inductor is not more than 0.5 volts or a total of approximately 1.5volts.

As the voltage applied to the digit electrode 16 reverses, the diode 28becomes reverse-biased to prevent clipping of the positive half of thecycle. Likewise, as the voltage applied to the selected segmentelectrode 20 reverses, the respective diode 32 prevents clipping of thenegative half of the cycle.

Since the selected digit electrode 16 and segment electrodes 20 aredriven out of phase with each other, the peak voltage across each of thetransistors, i.e., 26 and 30, is approximately one half the voltageacross the selected electrode pair or pairs. In the presently disclosedembodiment, the peak voltage across the transistors 26 and 30 isapproximately I75 volts each and thus the peak voltage across eachselected electrode pair is 350 volts, making 700 volts peak-to-peak.

In order to avoid excessive current and unnecessary energy losses, thetransistors 26 and 30 are preferably turned on during the half cycle inwhich the diodes 28 and 32 are back-biased. Thus, where f 25 khz, whichis a period of 40 ,us, the transistors 26 and 30 are preferably turnedoff for approximately 30 MS. Normally, in the present multiplexingsystem, the different transistors 26 are in conducting condition and areturned off sequentially in a re-occurring manner to sequentially biasthe different electrodes 16 to energized condition. Circuitry forperforming this function may be conventional and is therefore notdisclosed herein.

light emission. Also, the capacitance of each unselected electrode pairwill be constant.

It will be noted that the formation of each numeral display requiresfrom two to seven segment electrodes to be energized at a time.Therefore, if C equals the capacitance of a single electrode pair, and Nequals the number of digits in the display, then the total capacitanceseen by any transistor 30 is 2C for a selected digit and (N-l )C for allunselected digits or a total of (N+l )C. Since all segment electrodesare connected to the common line 29, the capacitance seen at thejuncture of the inductor 22 and electrode 16 is 2C for each drivensegment electrode plus C for each undriven segment electrode or 2PC(7-P)C 7+P)C where P equals the number of selected segment electrodes.Since P is never less than two or more than seven, the total range ofequivalent digit electrode capacitance is 9C to 14C, not including thatof the decimal point electrode 21. If the decimal point electrodecapacitance is 0.2C, the total range of digit capacitance is 9.2C to14.4C or an average of l 1.8C i 22 percent. Since a 22 percent variationin capacitance results in approximately a l 1 percent variation involtage, (V I V (L/C) and since the voltage applied to the selecteddigit electrode 16 is one half of the total voltage across a selectedelectrode pair, the voltage uncertainty due to variations in the numberof segment electrodes which are driven at any one time is approximately5 percent, which results in a negligible change in light intensity.

The rate of change of current through inductors 24 during the period T(FIG. 5) of a display cycle can be changed by changing the potential Vof the power supply 39a so that the energy lost per display cycle isindependent of whether a segment electrode is driven or not. That is,the potential V of power supply 39a is dictated by the losses or Q ofthe L-C combination (inductor 24-segment electrode 20) while thepotential V, of the power supply 39b may be of any convenient voltage.

For a given V and V the duty cycle or T /T -l-T may be computed from theexpression: Inductor current I V T V T /(T,+T )RVsw/R Where Vsw thevoltage across the series combination of transistors 30 an diodes 32 andR the resistance of inductors 24 Although any suitable form of polarityswitching device 39 may be employed to alternately apply differentvoltage potentials to the inductors 24 concurrently with turning off ofthe transistors 26 and 30, FIG. 4 illustrates one example of a devicesuitable for this purpose.

During the time interval T (FIG. 5), transistor 42 is biased on andtransistor 43 is biased off thus applying the positive voltage V to theline common to inductors 24 (e During the time interval T thecombination of transistor 43 and diode 44, provide a low impedancebidirectional current path from the junction of inductors 24 (e to thenegative potential V DESCRIPTION OF ALTERNATE EMBODIMENT FIG. 6illustrates a modified form of the invention wherein the drivecircuitry, including inductors 24, for the segment electrodes 20, aresimilar to those shown in FIG. 1. However, the inductors 22 for thedigit electrodes 16 are omitted and in lieu thereof, the unselecteddigit electrodes 16 are each connected through a normally on transistor50 and resistor 51 to ground potential. A second transistor 52 andresistor 53 are connected between the electrode 16 and a DC source 54 ofminus volts. The latter transistor 52 is normally biased to of conditionand the base thereof is connected to a signal input 55 through acapacitor 56. A diode 57 is connected between the electrode 16 andground to provide a low impedance path to ground during the positivehalf cycle of the segment drive. Resistors 53 and 51 limit currentthrough the electrode pairs to a level below that which may cause damageto the display.

Upon application of a positive pulse 60 to a selected signal input 55the respective transistor 50 is turned off and transistor 52 turned onto apply minus 175 volts to the corresponding electrode 16. Thus, theminus 175 volt potential, applied to thedigit electrode 16 will occurduring a positive half of the voltage waveform 49 (FIG. 5) applied tothe segment electrodes, resulting in a total peak voltage across thedigit and segment electrodes of 350 volts to cause emission.

While the circuitry of FIG. 6 may apply a DC component across theelectrode pairs, the duty cycle for the digit electrode drive circuitryis typically no more than 1% and therefore the DC component isnegligible.

I claim:

1. A drive circuit for a capacitive light emitting film display whereina phosphor film is interposed between a series of main electrodes on oneside and a series of groups of secondary electrodes on the other side,each of said groups being associated with a respective one of said mainelectrodes, comprising inductors connected in circuit with respectiveones of the electrodes of one of said series, power supply means forapplying an electrical potential for different ones of said mainelectrodes,

said power supply means including a group of main switching devicesconnected in circuit with respective ones of said main electrodes;

power supply means for applying an electrical potential to saidsecondary electrodes,

said last mentioned power supply means including a group of secondaryswitching devices connected in circuit with respective ones of saidsecondary electrodes whereby, when any of said main switching devicesand any of said secondary switching devices associated with a lastmentioned main switching device are simultaneously turned off, energystored in respective ones of said electrodes to cause light emission;

at least one of said groups of switching devices having a commonelectrical connection.

2. A drive circuit according to claim 1 wherein both of said groups ofswitching devices have respective common electrical connections.

3. A drive circuit according to claim 2 wherein both of said groups ofswitching devices have a common electrical connection.

4. A drive circuit for a capacitive light emitting film display whereina phosphor film is interposed between a main electrode on one side and aplurality of secondary electrodes on the other side, comprisinginductors connected in circuit with respective ones of said secondaryelectrodes,

power supply means for applying an electrical potential to saidsecondary electrodes,

said power supply means including a main switching device connected incircuit with said main electrode;

power supply means for applying an electrical potential to saidinductors,

said last mentioned power supply means including secondary switchingdevices connected in circuit with respective ones of said inductorswhereby, when said main switching device and any of said secondaryswitching devices are simultaneously turned off, energy stored inrespective ones of said inductors will be transferred to respective onesof said secondary electrodes to cause light emission;

said switching devices having a common electrical connection.

5. A drive circuit according to claim 4 comprising means for alternatelyapplying different voltage potentials to said common electricalconnection.

6. A drive circuit according to claim 4 comprising means for alternatelyapplying voltage potentials of different polarities to said commonelectrical connection.

7. A drive circuit according to claim 5 comprising means forconcurrently operating said last mentioned means and for turning offsaid switching devices.

8. A drive circuit for a capacitive light emitting film display whereina phosphor film is interposed between a main electrode on one side and aplurality of secondary electrodes on the other side, comprising a maininductor connected in circuit with said main electrode,

secondary inductors connected in circuit with respective ones of saidsecondary electrodes,

power supply means for applying an electrical potential to said maininductor, said power means including a main switching device connectedin circuit with said main inductor,

power supply means for applying an electrical potential to saidsecondary inductors,

said last mentioned power supply means including secondary switchingdevices connected in circuit with respective ones of said secondaryinductors whereby, when said main switching device and any of saidsecondary switching devices are simultaneously turned off, energy storedin respective ones of said inductors will be transferred to respectiveones of said electrodes to cause light emission,

said main and secondary switching means having a common electricalconnection.

9. A drive circuit according to claim 7 wherein said main inductor isconnected in series with said main electrode and said secondaryinductors are connected in series with respective ones of said secondaryelectrodes.

10. A drive circuit according to claim 8 wherein said main switchingdevice comprises a switching transistor connected to a pointintermediate said main inductor and said main electrode, and

said secondary switching device comprises switching transistorsconnected to points intermediate said secondary inductors and respectiveones of said secondary electrodes.

11. A drive circuit according to claim 10 comprising means forback-biasing said transistors.

12. A drive circuit according to claim 8 wherein said common electricalconnection is connected to said first and second power supply means.

13. A drive circuit according to claim 8 wherein said main inductor isconnected in circuit with said main electrode and said secondaryinductors are connected in circuit with respective ones of saidsecondary electrodes to form resonant circuits, said common electricalconnection being connected to a point intermediate said main inductorand said secondary switching devices.

14. A drive circuit for a capacitive light emitting film display whereina phosphor film is interposed between a plurality of main electrodes onone side and a plural ity of groups of secondary electrodes on the otherside, each of said groups being associated with a respective one of saidmain electrodes, comprising main inductors connected in circuit withrespective ones of said main electrodes,

secondary inductors connected in circuit with respective ones of saidsecondary electrodes,

power supply means for applying an electrical poten tial to differentones of said main electrodes,

said power supply means including main switching devices connected incircuit with respective ones of said main electrodes;

power supply means for applying an electrical potential to saidsecondary electrodes,

said last mentioned power supply means including secondary switchingdevices connected in circuit with respective ones of said secondaryelectrodes whereby, when any of said main switching devices and any ofsaid secondary switching devices associated with a said last mentionedmain switching device are simultaneously turned off, energy stored inrespective ones of said inductors will be transferred to respective onesof said electrodes to cause light emission;

said inductors having a common electrical connection.

15. A device according to claim 14 comprising means for alternatelyapplying different voltag'e potentials to said secondary inductors.

16. A device according to claim 14 comprising means for changing saidvoltage potential applied to said secondary inductors when saidswitching devices are turned off.

1. A drive circuit for a capacitive light emitting film display whereina phosphor film is interposed between a series oF main electrodes on oneside and a series of groups of secondary electrodes on the other side,each of said groups being associated with a respective one of said mainelectrodes, comprising inductors connected in circuit with respectiveones of the electrodes of one of said series, power supply means forapplying an electrical potential for different ones of said mainelectrodes, said power supply means including a group of main switchingdevices connected in circuit with respective ones of said mainelectrodes; power supply means for applying an electrical potential tosaid secondary electrodes, said last mentioned power supply meansincluding a group of secondary switching devices connected in circuitwith respective ones of said secondary electrodes whereby, when any ofsaid main switching devices and any of said secondary switching devicesassociated with a last mentioned main switching device aresimultaneously turned off, energy stored in respective ones of saidelectrodes to cause light emission; at least one of said groups ofswitching devices having a common electrical connection.
 2. A drivecircuit according to claim 1 wherein both of said groups of switchingdevices have respective common electrical connections.
 3. A drivecircuit according to claim 2 wherein both of said groups of switchingdevices have a common electrical connection.
 4. A drive circuit for acapacitive light emitting film display wherein a phosphor film isinterposed between a main electrode on one side and a plurality ofsecondary electrodes on the other side, comprising inductors connectedin circuit with respective ones of said secondary electrodes, powersupply means for applying an electrical potential to said secondaryelectrodes, said power supply means including a main switching deviceconnected in circuit with said main electrode; power supply means forapplying an electrical potential to said inductors, said last mentionedpower supply means including secondary switching devices connected incircuit with respective ones of said inductors whereby, when said mainswitching device and any of said secondary switching devices aresimultaneously turned off, energy stored in respective ones of saidinductors will be transferred to respective ones of said secondaryelectrodes to cause light emission; said switching devices having acommon electrical connection.
 5. A drive circuit according to claim 4comprising means for alternately applying different voltage potentialsto said common electrical connection.
 6. A drive circuit according toclaim 4 comprising means for alternately applying voltage potentials ofdifferent polarities to said common electrical connection.
 7. A drivecircuit according to claim 5 comprising means for concurrently operatingsaid last mentioned means and for turning off said switching devices. 8.A drive circuit for a capacitive light emitting film display wherein aphosphor film is interposed between a main electrode on one side and aplurality of secondary electrodes on the other side, comprising a maininductor connected in circuit with said main electrode, secondaryinductors connected in circuit with respective ones of said secondaryelectrodes, power supply means for applying an electrical potential tosaid main inductor, said power means including a main switching deviceconnected in circuit with said main inductor, power supply means forapplying an electrical potential to said secondary inductors, said lastmentioned power supply means including secondary switching devicesconnected in circuit with respective ones of said secondary inductorswhereby, when said main switching device and any of said secondaryswitching devices are simultaneously turned off, energy stored inrespective ones of said inductors will be transferred to respective onesof said electrodes to cause light emission, said main and secondaryswitching means having a common electrical connection.
 9. A drivecircuit according to claim 7 wherein said main inductor is connected inseries with said main electrode and said secondary inductors areconnected in series with respective ones of said secondary electrodes.10. A drive circuit according to claim 8 wherein said main switchingdevice comprises a switching transistor connected to a pointintermediate said main inductor and said main electrode, and saidsecondary switching device comprises switching transistors connected topoints intermediate said secondary inductors and respective ones of saidsecondary electrodes.
 11. A drive circuit according to claim 10comprising means for back-biasing said transistors.
 12. A drive circuitaccording to claim 8 wherein said common electrical connection isconnected to said first and second power supply means.
 13. A drivecircuit according to claim 8 wherein said main inductor is connected incircuit with said main electrode and said secondary inductors areconnected in circuit with respective ones of said secondary electrodesto form resonant circuits, said common electrical connection beingconnected to a point intermediate said main inductor and said secondaryswitching devices.
 14. A drive circuit for a capacitive light emittingfilm display wherein a phosphor film is interposed between a pluralityof main electrodes on one side and a plurality of groups of secondaryelectrodes on the other side, each of said groups being associated witha respective one of said main electrodes, comprising main inductorsconnected in circuit with respective ones of said main electrodes,secondary inductors connected in circuit with respective ones of saidsecondary electrodes, power supply means for applying an electricalpotential to different ones of said main electrodes, said power supplymeans including main switching devices connected in circuit withrespective ones of said main electrodes; power supply means for applyingan electrical potential to said secondary electrodes, said lastmentioned power supply means including secondary switching devicesconnected in circuit with respective ones of said secondary electrodeswhereby, when any of said main switching devices and any of saidsecondary switching devices associated with a said last mentioned mainswitching device are simultaneously turned off, energy stored inrespective ones of said inductors will be transferred to respective onesof said electrodes to cause light emission; said inductors having acommon electrical connection.
 15. A device according to claim 14comprising means for alternately applying different voltage potentialsto said secondary inductors.
 16. A device according to claim 14comprising means for changing said voltage potential applied to saidsecondary inductors when said switching devices are turned off.